One of the essential steps in navigating a bone and tools with MEMS sensors is to initially locate the bone relative to the sensors, i.e., creating a frame of reference or coordinate system. Some steps must be performed to create the frame of reference considering specifications of MEMS sensor systems. Specifications of MEMS sensor systems may include orientation tracking along two degrees of freedom only, or the absence of positional tracking. Known steps of calibration comprise various manipulations of a sensor and/or bone, for the orientational setting of the sensor (hereinafter, the reference tracker) with respect to the bone. Once the orientational setting is completed, navigation steps may be performed, with the bone being tracked via the frame of reference using the reference tracker.
In some instances, the sensor must be constrained with respect to a bone for subsequent tracking. For femur tracking for example, the orientation of the sensor relative to the lateral axis can be constrained mechanically (e.g., with claws inserted under the posterior condyles) so that the sensor lateral axis is aligned with the lateral axis of the bone.
In other instances, various tools used to perform alterations on a bone must be calibrated with respect to a MEMS reference tracker, to be tracked during navigation. One example is the cutting block (a.k.a., positioning block), which may be mechanically constrained to the MEMS reference tracker for the calibration to be made. In such known cases, specific manipulations must be executed by the operator to ensure that the positioning block is connected to the reference tracker for the calibration of the positioning block, for subsequent tracking and bone alterations.
Patient specific instrumentation (hereinafter “PSI”) pertains to the creation of instruments that are made specifically for the patient, and that hence have a contact surface(s) that is a negative of the bone surface to which it will be anchored. Hence, when the contact surface of the PSI is positioned against the bone, there is complementary contact (the contact surface negatively matching the anchor surface). PSI are typically manufactured from data using imagery to model bone geometry and thus be a true negative. The complementary engagement is predictable as such contact surfaces are specifically manufactured to match the surface of a bone. It would therefore be desirable to use PSI technology with MEMS.